Canker Sore vs Cold Sore: How to Tell Them Apart, and What Helps

Canker sore vs cold sore: the fastest way to tell them apart is where the sore is. Canker sores form inside your mouth on the soft, moving tissue and are not contagious, while cold sores usually appear on or around your lips, start as a cluster of small blisters and are caused by a contagious virus.

The two get mixed up all the time, partly because people use the names loosely and partly because both hurt. But they have different causes, they spread differently, and they need different care. Below I explain how to recognize each one, what tends to trigger them, what actually helps, and when a sore needs to be looked at in person.

Canker sore vs cold sore: the quick comparison

When patients ask me about a canker sore vs cold sore, I look at three things: where the sore is, what it looked like at the start, and whether it has come back in the same spot before. If you only read one part of this article, make it this table:

Doctors and dentists also call canker sores aphthous ulcers. If you have heard the term mouth ulcers, it usually refers to the same kind of sore inside the mouth.

What a canker sore is, and why you get them

A canker sore is a small, shallow ulcer in the lining of the mouth. It is not an infection you caught from someone, and you cannot pass it on by kissing or sharing a drink. Most are smaller than a pencil eraser, but they can be surprisingly painful because they sit on tissue that moves every time you talk or eat.

Nobody knows the single cause, but certain things make them more likely:

• Minor injury: biting your cheek, a sharp tooth edge, braces, a toothbrush slip or a dental appliance rubbing.
• Irritating ingredients: some people react to sodium lauryl sulfate (SLS), the foaming agent in many toothpastes.
• Stress and poor sleep: many people notice sores during busy, stressful stretches. Our article on stress and oral health explains the connection.
• Foods: acidic or spicy foods can trigger or worsen them in some people.
• Hormonal changes and a family tendency.
• Nutritional and medical factors: low iron, vitamin B12 or folate, and conditions such as celiac disease or inflammatory bowel disease. Our guide to celiac disease and oral health covers the mouth signs to know.

The canker sore vs cold sore difference matters here, because the triggers overlap (stress shows up on both lists) but the cause does not. A canker sore is your own tissue reacting; a cold sore is a virus waking up. If you get canker sores often, keeping a simple note of when they appear can help you and your dentist or physician spot a pattern.

What a cold sore is, and why it comes back

A cold sore, also called a fever blister, is caused by the herpes simplex virus. Most people pick up the virus in childhood through close contact, and many never know they carry it. After the first infection, the virus stays dormant in nerve cells and can reactivate from time to time.

A cold sore usually follows a familiar pattern: a tingling or burning spot on the lip, then a small cluster of blisters, then weeping and a crust that falls off as the skin heals. Common triggers for a flare include:

• A cold, flu or fever
• Strong sun or wind on the lips
• Stress and fatigue
• Hormonal changes
• Injury to the lip, including some dental work

Cold sores mostly appear outside the mouth. Occasionally the virus causes sores on the firm tissue inside, such as the gums or the roof of the mouth, which is one reason a sore that doesn't fit the usual pattern is worth having checked.

Are they contagious?

Canker sores are not contagious. You don't need to avoid people, and you can't spread them to another part of your mouth.

This is the most practical part of the canker sore vs cold sore question, because it changes how you act around other people. Cold sores are contagious, most of all from the first tingle until the crust has fully fallen off. While you have one:

• Avoid kissing and sharing cups, utensils, lip balm or towels.
• Don't touch the sore, and wash your hands if you do.
• Keep it away from babies and anyone with a weakened immune system.
• Be careful not to touch your eyes after touching the sore.

If you have a dental appointment while a cold sore is active, let the office know. Many dentists prefer to reschedule non-urgent work, because stretching the lips can make the sore worse and spread the virus.

Does SLS cause canker sores?

Sodium lauryl sulfate (SLS) is the detergent that makes toothpaste foam. It doesn't cause canker sores on its own, but in people who are prone to them it can irritate the lining of the mouth and may trigger new sores or make existing ones sting more. The research is mixed, and not everyone notices a difference.

Because switching is easy and harmless, I suggest an SLS-free toothpaste to anyone who gets canker sores often. Check the ingredient list for "sodium lauryl sulfate"; many brands make SLS-free versions. If you also want prescription-strength cavity protection, PreviDent 5000 Dry Mouth is an SLS-free option that a dentist can approve.

Give it a few weeks before you judge it, since canker sores come and go on their own schedule.

SLS doesn't play a role in cold sores, so switching toothpaste won't prevent those. For cold sores, the triggers to manage are sun, illness and stress.

What helps a canker sore feel better

Most canker sores settle on their own, so the goal is comfort and avoiding anything that keeps irritating the spot:

• Switch to an SLS-free toothpaste and brush gently around the sore with a soft brush.
• Use a gentle, alcohol-free rinse. Alcohol rinses and strong mint flavors often sting. A rinse like CloSYS Sensitive is alcohol-free and comes in an unflavored version for sensitive mouths.
• Avoid triggers while it settles: acidic, spicy, salty or crunchy foods.
• Ask a pharmacist about protective or numbing gels that cover the sore while you eat.
• Look for a rough edge on a tooth, filling or appliance that keeps catching the same spot, and have it smoothed.

Skip strong antiseptic rinses such as chlorhexidine on an open sore unless your dentist tells you to use one. For a wider view of sores, spots and lumps in the mouth, see our guide to bumps in the mouth.

What helps a cold sore

Cold sores are a skin and virus problem, so the most effective treatment comes from a pharmacist or physician rather than a dental product. Antiviral creams and tablets work best when started at the first tingle, which is why people who get frequent cold sores often keep treatment on hand. A physician can also discuss preventive options if you get them often.

Simple steps help too: a lip balm with sunscreen if sun is your trigger, keeping the area clean and dry, and leaving the crust alone so it can heal without scarring.

See a physician promptly if a cold sore is near your eye, if sores spread widely, if you have a weakened immune system, or if a sore hasn't healed in about two weeks.

Questions people ask

Sources

• ADA MouthHealthy: Canker sores
• ADA MouthHealthy: Cold sores
• Cleveland Clinic: Canker sores
• NHS: Cold sores